Trunking communication systems are known to comprise a plurality of communication units arranged into a predetermined number of communication groups, a limited number of communication resources that are transceived via a predetermined number of repeaters, and a communication resource allocator that allocates the limited number of communication resources among the plurality of communication units. The communication resource may be a time division multiplex (TDM) slot(s), a frequency carrier, a pair of frequency carriers, or any other radio frequency (RF) transmission means. Of the communication resources, one is typically selected as a control channel while the remaining communication resources act as voice channels. The control channel transceives outbound signalling words (OSW) from the communication resource allocator to the communication units and also inbound signalling words (ISW) from the communication units to the communication resource allocator. OSWs and ISWs are data which indicate system operations to be performed or system features that are desired to be accessed by the communication units. For example, when a communication unit desires to transmit a message to another communication unit, it transmits an ISW to the communication resource allocator requesting access to a communication resource and identifies the target communication unit. If the communication resource allocator grants the communication units request, it transmits the OSW to the communication unit indicating to both the initiating communication unit and the target communication unit which communication resource the conversation will take place on.
At times, the communication resource allocator will not grant the communication unit's request for a communication resource primarily due to an overloading of the system, i.e. all of the communication resources are allocated and there are several requests waiting to be granted. Under such conditions, the communication resource allocator, which typically includes a queue to store the request that cannot be granted upon receipt, is saturated and cannot handle, for that moment, another request for a communication resource. To handle this overload condition, a prior art system may temporarily use the control channel as a voice channel thus increasing the number of voice channels which in turn reduces the communication resource request backlog. When the control channel is acting as a voice channel, the communication resource allocator cannot receive ISWs or transmit OSWs which leaves communication units that are not presently in a communication or the communication resource request queue temporarily incapable of communicating with the communication resource allocator. This presents a serious problem when a communication unit needs to immediately communicate with the communication resource allocator, for example, when the unit has an emergency condition.
One approach to overcome this problem is to select one of the communication resources to act as a voice/control channel when the actual control channel is being used as a voice channel. In such a system, the communication units may transmit a select number of control functions to the communication resource allocator via the control/voice channel. The control information is transmitted in a low frequency signalling manner, i.e. frequency below 300 hertz. This overcomes the problem of communication units not being able to access the communication system when the control channel is being used as a voice, however, this approach limits the access to a select number of control functions that can be executed. Therefore, a need exists for a method that allows communication units to transmit any control information to the communication resource allocator when the control channel is being used as a voice channel.